This application claims benefit of Japanese Patent Application No. 11-363686 filed on Dec. 22, 1999, the contents of which are incorporated by the reference.
The present invention relates to mobile communication systems and simplified inter-base station synchronizing methods used for the same and, more particularly, to an inter-base station synchronizing method in cellular communication adopting CDMA (Code Division Multiple Access) system.
In cellular communication adopting CDMA system, usually a technique of frequency and timing synchronization of base and mobile stations is important for maintaining a constant communication quality. For example, in the case of adopting IS-95, which is adopted in North America, inter-base station synchronization is obtained by using GPS (Global Positioning System) at chip clock level. In this case, once a mobile station establishes timing synchronization with a base station, it can relatively readily establish timing synchronization with a different base station by using the established timing, although the readiness vary with signal propagation channels.
In the meantime, in W-CDMA (Wide band-CDMA) system which is a positively promising third-generation digital cellular system for future adoption, inter-base station synchronization at chip clock level is not guaranteed. In this system, the mobile station should therefore repeatedly perform the same procedure as initial synchronization capturing process to obtain timing synchronization with a different base station. That is, the system poses in such case a problem that a certain time is necessary until the synchronization capturing.
Another technical feature of the cellular communication adopting the CDMA system is diversity hand-over. In diversity hand-over technique, down signals from different base station or different sectors, irrespective of the same base station, are transmitted with the same data provided therein, and a mobile station simultaneously receives and combines such down signals. This technique can improve the signal transmission quality. Another advantage of the technique is that radio transmission without momentary interruption is possible during movement of the mobile station from an area covered by one base station or sector to another.
However, since the W-CDMA system does not guarantee the inter-base station synchronization at chip clock level as noted above, the mobile station can not directly efficiently receive and combine signals. Some measure, therefore, is necessary to solve this problem.
By way of example, ARIB (Association of Radio Industries and Businesses) provides prescriptions, which have not yet been standardized, propose the following procedure in xe2x80x9cSpecification of Air-Interface for 3G Mobile System Volume 3 (Ver 1.0)xe2x80x9d.
In xe2x80x9c3.2.6.6.1. Diversity Hand-over (intra/inter-cell)xe2x80x9d in xe2x80x9c3.2.6.6 Hand-overxe2x80x9d, a mobile station measures the timing difference between communicating cell and hand-over destination perch channel, and reports the measured timing difference to BSC (Base Station Controller) via base station. The BSC controls the down transmission timing of the hand-over destination base station according to the notified difference to be substantially the same as such hand-over destination timing in the mobile station.
In the prior art W-CDMA system as described above, the mobile station should take the above ARIB prescription procedure for efficient signal reception and combination. Such procedure, although necessary for synchronization among out-of-synchronization base stations, is disadvantageous because of the facts that it is intrinsically unnecessary so long as down signal timing synchronization prevails and that it increases unnecessary signaling.
An object of the present invention, therefore, is to provide a mobile communication system and a simplified inter-base station synchronizing method used for the same, which can solve the problems discussed above, and permits search time reduction and suppression of unnecessary signaling at hand-over occasions.
According to an aspect of the present invention, there is provided a mobile communication system comprising a plurality of base stations asynchronous to one another, a controller for controlling each of the plurality of base stations and a mobile station for receiving down signals from and transmitting up signals to the plurality of base stations; the mobile station having a means for detecting the difference of timings of reception of down signals from the plurality of base stations, and a means for transmitting the detected reception timing difference to a reference base station; the plurality of base stations each having a means for determining a own distance function with respect to the mobile station by detecting a time interval from the timing of down signal transmission till the timing of up signal reception in the mobile station, a means for transmitting the determined own distance function to the controller, and a means for transmitting the detected reception timing difference from the mobile station to the controller upon reception of a report from the controller that the own base station is the reference base station; the controller having a means for calculating the difference of timings of down signal transmission from the reference and other base stations on the basis of the distance functions thereof and the detected reception timing difference form the mobile station, and a means for transmitting the calculated transmission timing difference to the other in-question base station, thus causing down signal transmission timing updating.
The mobile station transmits the up signal a predetermined period of time after the pertinent down signal reception timing. The plurality of base stations each determine the own distance function in a statistical process with values received from the mobile station in a predetermined constant period of time. The plurality of base stations each determine the own distance function in a statistical process with values received from a plurality of mobile stations. The plurality of base stations each cause the down signal transmission timing updating within a timing holding capacity of the mobile station.
According to another aspect of the present invention, there is provided a simplified inter-base station synchronizing methods used for a mobile communication system which comprises a plurality of base stations asynchronous to one another, a controller for controlling each of the plurality of base stations and a mobile station for receiving down signals from and transmitting up signals to the plurality of base stations; wherein the mobile station includes a step for detecting the difference of timings of reception of down signals from the plurality of base stations, and a step for transmitting the detected reception timing difference to a reference base station; the plurality of base stations each includes a step for determining a own distance function with respect to the mobile station by detecting a time interval from the timing of down signal transmission till the timing of up signal reception in the mobile station, a step for transmitting the determined own distance function to the controller, and a step for transmitting the detected reception timing difference from the mobile station to the controller upon reception of a report from the controller that the own base station is the reference base station; and the controller includes a step for calculating the difference of timings of down signal transmission from the reference and other base stations on the basis of the distance functions thereof and the detected reception timing difference form the mobile station, and a step for transmitting the calculated transmission timing difference to the other in-question base station, thus causing down signal transmission timing updating.
The mobile station transmits the up signal a predetermined period of time after the pertinent down signal reception timing. The the plurality of base stations each determine the own distance function in a statistical process with values received from the mobile station in a predetermined constant period of time. The plurality of base stations each determine the own distance function in a statistical process with values received from a plurality of mobile stations. The plurality of base stations each cause the down signal transmission timing updating within a timing holding capacity of the mobile station.
According to other aspect of the present invention, there is provided a mobile communication system in which each of the plurality of base stations asynchronous to one another and a mobile station are controlled for receiving down signals from and transmitting up signals to the plurality of base stations by a controller, wherein the mobile station is adapted to detect the difference of timings of reception of down signals from a reference base station and an other in-question base station among the plurality of base stations and transmit the detected reception timing difference to the reference base station, the plurality of base stations are each adapted to determine a own distance function with respect to the mobile station by detecting a time interval from the timing of down signal transmission from the mobile station till the timing of up signal reception in the mobile station, transmit the determined own distance function to the controller and transmit the detected timing difference from the mobile station to the controller upon reception of a report from the controller that the own base station is the reference base station, and the controller is adapted to calculate the difference of timings of down signal transmission from the reference and other base stations from the distance functions thereof and the detected reception timing difference from the mobile station and transmit the calculated transmission timing difference to the other base station for updating the down signal transmission.
More specifically, the simplified inter-base station synchronizing method according to the present invention is one, which permits easily obtaining synchronization among out-of-synchronization W-CDMA system base stations. As a method of determining inter-base station timing error, is assumed a procedure such as ARIB procedure mentioned in xe2x80x9c3.2.6.1. Diversity Hand-over (intra/inter-cell)xe2x80x9d in xe2x80x9cSpecifications of Air-Interface for 3G Mobile System Volume 3 (Ver 1.0)xe2x80x9d.
This procedure permits the BSC to measure inter-base station chip timing error in each mobile station as reported from each mobile station. This measurement, however, involves errors due to the distance between the mobile and base stations, due to signal propagation channel status, for instance environmental error introduced when the measurement is executed in a multiple pass status, due to accuracy of measurement in the mobile station and so forth. Therefore, the measured error can not be directly dealt with as inter-base station error.
It is conceivable to use, for canceling error due to distance, values which can be measured in the base station. The mobile station is adapted to transmit up signal after a delay time of, for instance, one half slot from the down signal reception for satisfying a certain time relation between up and down signal timings at all times. This means that the error between the actual reception timing and rated signal reception timing (reception timing to be received) of the base station concerning the signal transmitted from the mobile station, is thought to correspond to double the distance between the base and mobile stations. Thus, by measuring the error and reporting the measured error to the BSC at an adequate cycle interval, the BSC can cancel the effects of the error by calculation.
Other errors may be corrected in statistical processes with values obtained by distance cancellation, and it is possible to obtain probable values to certain extents. The timings of the base stations under control by the BSC, thus can be readily synchronized by causing gradual base station down signal timing updating.
While heretofore very long search time was required with asynchronous base stations, by readily taking inter-base station synchronization using inter-base station timing difference measurements obtained by mobile station, it is possible to reduce search time and also reduce inter-base station signaling. That is, once inter-base station synchronization is established to a certain extent, the mobile station can search a new base station in a reduced search time, and also it is possible to preclude unnecessary signaling in hand-over occasions.
Other objects and features will be clarified from the following description with reference to attached drawings.